1. Field of the Invention
The present invention relates to a method and apparatus for producing numerically correct partial stacks from individual leaves or sheets interfolded in a U-shape or a zigzag form on an interfolder. A double stream is formed from one or more streams of sheets by staggering or guiding together the streams. This double stream is then continuously folded in a zigzag form and is piled up in a continuously growing stack, from which partial stacks of a specific size are separated. Gaps are formed in the streams of sheets which simplify the introduction of separating elements into the stack. For this purpose, sheets are removed from the streams of sheets and optionally placed accurately in position on successive sheets.
2. The Prior Art
An apparatus of this type is known to those skilled in the art as an interfolder and have been known for a long time. They function by folding material into U-shaped or zigzag-shaped folds, and for producing stacks of folded sheet material. During this method of operation, one common stream of sheet material is formed from one or more, preferably two, prepared lengths of material which each consists of an uninterrupted series of sheets of equal length which are separated from one another or are attached to one another by a perforation. The single stream of sheet thus produced is then folded in a zigzag format and is then piled up in a stack.
Depending upon the number of folds per sheet, the common stream of sheet material is produced either by staggering the prepared lengths of material or by guiding them together in an offset manner. Most common are interfolders which process two lengths and produce sheets which have only one fold and are placed on one another in a U shape. In this type, the sheets of the two lengths of material are placed one on top of the other in such a way that they are staggered with respect to one another by half a sheet in length.
In order to make full use of the productivity of the interfolders of the prior art type, it is necessary for them to be followed by a semi-automatic or fully-automatic packaging arrangement. This gave rise to the requirement for the sheet stacks which are continuously produced by the zigzag folding to be divided into numerically correct partial stacks which are then packaged. For this purpose, depending upon the machine cycle and the predetermined lot size of the partial stacks, separating or supporting elements are introduced from the side into the sheet stack and a partial stack is separated from the sheet stack. In order to achieve the most accurately possible division of the sheet stack into numerically correct partial stacks, it is advisable to carry out separation directly upon formation of the stack; that is, immediately downstream from the folding rollers. Separations of this type generally operate satisfactorily. However, problems occur at the moment when the downstream front sheet of the sheet stack and the upstream rear sheet of the partial stack, which are interfolded, are to be separated without disruption. This then leads to difficulties and disruptions, particularly when partial stacks with small lot sizes have to be formed at high production capacities by the interfolder.
In order to solve problems of this type, there have already been proposals, such as in U.S. Pat. No. 4,717,135 and in EP-O 291,211 A 2, in which small gaps are created by folding back advancing sheet parts onto the sheet itself in the individual sheet streams. In this way, interfolding of the sheets corresponds to a plane of separation, so that the actual separating operation is simplified.
However, it has been found in practice with modern high-capacity interfolders that this small gap is not sufficient for a simple separating operation. Even a high expenditure on control technology and machinery which is urgently required as a result does not guarantee reliable separation of numerically correct partial stacks. Folding back, or round folding, of several sheets, which might be proposed as a possible solution, cannot be carried out. The material thickness produced in these operations with the combined lengths of material is limited by the width of the folding gap between the folding rollers, since this needs to be as narrow as possible in order to achieve a good quality fold. In addition, this possible solution has the inherent danger that when several sheets are folded back, or folded round, they are shifted out of register with respect to one another and to the rest of the series of sheets. This would inevitably lead to disruptions in the taking up, the transporting, and the zigzag folding.